1. Field of the Invention
This invention relates to a process for forming a plastic article from a plurality of layers of thermoplastic material. More, particularly, at least one layer of the plurality of layers is heated separately to a temperature not to exceed the melting temperature of any thermoplastic material in the layer to be heated and the plurality of layers is stacked, prior to forming the stacked plurality of layers into a plastic article. The plurality of layers of thermoplastic material may be diverse, or may be similar and stacked so as to provide a layered angular orientation prior to forming.
2. Description of the Prior Art
U.S. Pat. No. 3,739,052 to Ayres et al. describes a scrap free process for rapidly making containers from a multilayered or homogeneous thermoplastic blank. This blank is preheated, immediately forged into a preform, and then thermoformed into a container.
U.S. Pat. Nos. 4,323,531 and 4,352,766 to Bradley et al., teach a process for making plastic articles from resinous powders. The resinous powders are initially compressed into briquettes which may be homogeneous, blended or multilayered and may contain a chemical blowing agent. The briquette is then heated to a temperature in the range from about the alpha transition temperature to less than the melt temperature of the polymer to somewhat soften and sinter the briquette. The sintered briquette is then useful as a blank in an essentially scrap-free, substantially solid phase, relatively low temperature process and may be forged into a preform and subsequently, or at a later time, thermoformed into a plastic article.
U.S. Pat. No. 4,510,108 to Cleereman et al., teach two additional methods for preparing a compressed resinous powder briquette useful as a blank in an essentially scrap-free, solid phase process. It is an improvement of the process disclosed in U.S. Pat. No. 4,323,531.
However several particular problems still exist in such a process when utilizing a single thermoplastic blank or a single compressed powder briquette of some homogenous materials or multilayered diverse materials.
A first problem with some single blank or briquette of multilayered diverse materials is forming viscosity. Individual resins, due to molecular architecture, have specific viscosity versus temperature curves and thus a specific viscosity-temperature range in which thermoforming of the resin may occur. Multilayer sheet blanks or multilayer compressed powder briquettes having at least two layers of diverse materials may encounter forming viscosity problems. If the forming viscosities of the diverse resins are similar, "plug flow" (i.e., flow approaching the uniform velocity profile of a cross-section) of the multilayer blank or briquette will take place so that partial or complete disruption of the multilayer blank or briquette does not take place. If, however, the forming viscosities of the diverse resins in the multilayer blank or briquette are not similar, partial or complete disruption of the multilayer blank or briquette layer structure may occur due to nonuniform flow of one or more layers of the diverse resins.
U.S. Pat. No. 3,739,052 states that multilayer blanks or briquettes, of two or more diverse resins, will have different forming temperatures and softening points than their individual layers of diverse resins. Thus, an optimum forming temperature must be determined for a multilayer blank or briquette. It has been found according to U.S. Pat. No. 3,739,052 that the forming temperature of a multilayer blank or briquette is ordinarily dominated by the forming temperature of the layer in contact with the forming surface. Sometimes though, due to widely divergent forming viscosities, an optimum temperature for a multilayer blank or briquette may not be achievable due to deleterious effects to one or more of the layers of diverse resin material. These deleterious effects may include, loss of barrier properties, loss of physical or mechanical properties, color degradation or inability to fabricate.
A second problem with a single blank or briquette of multilayered diverse materials is that of resin degradation. This degradation, be it in color, barrier or mechanical properties, can be extremely severe. Anytime a single multilayer blank or briquette of diverse materials is heated for a specific time and at a specific temperature to reach an optimum processing temperature for the single multilayer blank or briquette and that specific heating time, temperature or combination of time and temperature exceeds an allowable heating time, temperature or combination of time and temperature for any one of the diverse materials, that material will degrade to some extent. For example, when using SARAN resin (a vinylidene chloride based homopolymer or copolymer) with a material having a substantially higher processing temperature than SARAN resin, for example polycarbonate, in a single multilayer blank obtained for example, by coextrusion or lamination, severe degradation will result to the SARAN resin due to the heating time, temperature or combination of time and temperature necessary to reach a processing temperature for a single multilayer blank of polycarbonate and SARAN resin. While SARAN resin is particularly sensitive to heat degradation, other resins can experience similar effects, differing only by degree.
Producing a plastic article in the above manner imposes a need to heat the single multilayer blank or briquette twice, or at the very least, for a period of time exceeding that of the present invention. A first heating is required to produce the single multilayer blank or briquette and a second heating, or at the very least, an extended time of the first heating is required before forming the single multilayer blank or briquette into a plastic article.
Single multilayer blanks or multilayer briquettes may not be able to overcome molecular architecture constraints when utilizing some combinations of diverse resins in a single layered blank. These molecular architecture constraints include forming viscosity and resin degradation. A single thermoplastic blank of homogenous material or an oriented material (e.g., fiber-oriented) may also encounter problems of nonuniform orientation. A uniform circumferential orientation is desirable to avoid problems with dimensional stability, nonuniform physical properties and material distribution.
Thus, there exists a need for a process to overcome these problems.